In communication systems information comprising speech or other data is transmitted via a high frequency signal. For these communication systems communication signals are modulated from the base band onto a carrier signal via phase modulation, amplitude modulation or other modulation methods within a transmitter.
In general, at first a base band is generated comprising the information being transmitted. For increasing the transfer rate it is advantageously to use the amplitude component as well as the phase component as information carriers. After modulating the signal onto a carrier signal the signal may be amplified to a proper transmitting power.
It is very important to reduce power consumption of the used transmitters. In particular, the used power amplifiers, like class A or B amplifiers, dissipate much energy. Thus one method to realize reduction of power consumption is to make use of power amplifiers having a high efficiency, like switching amplifiers. For instance, class D or class E amplifiers can be used.
However, for employing switching amplifiers, suitable modulation methods are necessary since the switching amplifiers make high demands to linearity of the signal being amplified. Due to these high demands it is not possible to amplify a modulated signal which does not comprise a constant envelope directly. Thus appropriate modulation methods are needed, for instance, pulse width modulation (PWM) or pulse density modulation (PDM). However, these modulation methods encompass the disadvantage of a limited dynamic range.
For controlling a switching amplifier, several methods are known. According to one method, the input of the amplifier is driven with a two-level PWM signal, which is generated by comparing the modulated carrier signal with a triangular or saw tooth signal, which fundamental frequency is at least twice the carrier frequency.
Another possibility for driving a switching amplifier is to use a PWM signal which is generated by a band pass sigma-delta. However, these solutions offer the drawback of loss in bandwidth of the processed envelope modulation.
According to another class of solutions, the amplitude component is added via modulation of the supply voltage of the switching power amplifier. This modulation can be done via an efficient DC-DC converter. For instance, a DC-DC converter is combined with a linear regulator. However, it is difficult to achieve a high modulation bandwidth and to suppress unwanted noise or ripple generated by the DC-DC converter at the same time.
It is one object of the present application to provide reduced power consumption of the apparatus. A further object is to reduce unwanted noise and ripple generated by the DC-DC converter. A further object is to increase the dynamic range of the apparatus.